The combustion process in the internal combustion engine of a vehicle is never perfect. Harmful emissions that result from the incomplete combustion are carbon monoxide, unburned hydrocarbons and NOx. There are existing and future emission standards for such gaseous emissions. Carbon monoxide and hydrocarbons are typically removed from the exhaust gas of the internal combustion engine by use of an oxidation catalyst as part of a catalytic convertor. In simple terms a catalytic convertor needs to provide a structure that exposes a maximum surface area of catalyst to the exhaust gas stream and the catalyst needs to aid the reaction of the carbon monoxide and hydrocarbons with oxygen in the exhaust gas stream. In this structure the harmful gases are converted into what are considered less harmful materials i.e. carbon dioxide and water.
Carbon dioxide itself is one of the main gaseous emissions from the combustion process within an internal combustion engine, when complete combustion occurs. It is considered by many in the scientific community to be a major factor in global warming as a result of increase of greenhouse effect due to its emissions. Currently in Europe there are no mandatory emission standards for carbon dioxide emitted from passenger cars, but voluntary agreements with motor manufacturers are in place. In the UK financial measures have been introduced in 2001 in an attempt to lower carbon dioxide emissions by linking vehicle excise duty to carbon dioxide emission levels and type of fuel used. Therefore passenger car buyers pay lower annual vehicle excise duty for vehicles that emit lower levels of carbon dioxide.
However the European Union has now agreed a mandatory carbon dioxide target for passenger car manufacturers. Under this legislation car manufacturers have a fleet average emissions target for vehicles sold in Europe of 130 gCO2/Km or below by 2015. This target is being gradually phased in from 2012. The US also recognises that the running of vehicles provides a major source of carbon dioxide emissions. Greenhouse Gas (GHG) emission standards have been set by the Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHTSA). These reductions are being phased in from 2009 to 2016 and are defined in terms of CO2— equivalents (gCO2/mile) whereby emissions of N2O and CH4 are included with multiplying factors of 296 and 23 respectively. Therefore car manufacturers are striving to lower their carbon dioxide emissions by a variety of measures such as weight reduction, variable valve actuation, low friction components and stop-start technologies. By adopting such measures a reduction in CO2 emissions of 25-30% is reportedly achievable. Many of these measures will also lead to a reduction in the exhaust gas temperature because of the improved fuel efficiency.
Although there have been moves across the world to reduce the level of sulphur present in diesel fuel (currently the mandatory level in Europe is 10 ppm) sulphur poisoning of exhaust gas catalyst systems is still an issue. For example ceria (cerium dioxide), which can be used as a support and/or catalyst promoter, is known to be thermally stable but it is susceptible to sulphur poisoning. Palladium is also well known for its ability to readily react with sulphur dioxide to form a stable sulphate. One way of removing sulphur from the catalyst is regeneration at higher temperatures in order to decompose catalyst metal sulphates that may have formed. This is becoming increasingly more difficult as many of the above measures for reduction in CO2 emission will also lead to a reduction in the exhaust gas temperature. For example the decomposition of palladium sulphate in a lean environment requires temperatures in excess of 700° C. or lower temperatures in rich fuel gas exhaust but then there is a fuel penalty because of the creation of the rich environment.
The catalytic activity of a convertor, i.e. its ability to react with exhaust emissions, is very low at vehicle start up because the catalyst must be heated to its light off temperature before it is fully active. For carbon monoxide and hydrocarbons the light off temperature is recognised as the temperature at which carbon monoxide or hydrocarbon conversion efficiency reaches 50%. Therefore it is important to have as low a light off temperature as possible. For some systems the process of heating to the light off temperature can take up to a couple of minutes. Close coupled catalysts have been employed upstream of the catalytic convertor and hence closer to the exhaust emission to remedy this problem as the close coupled catalyst can reach its light off temperature in seconds. However there are some disadvantages to this approach as close coupled catalysts are thought to be less effective at converting the harmful gases and may be more susceptible to poisoning from exhaust contaminants.
The cost of the catalyst must be minimised, for example by using less materials and/or using less expensive materials. Catalysts that have been used most successfully for oxidation reactions in catalytic convertors are precious metals, particularly platinum, which is a very expensive material. Palladium has been combined with platinum to reduce the catalyst costs and also has been found to reduce sintering of the platinum at higher temperatures. However palladium itself is known to have lower reactivity under very oxidising (lean oxidising) conditions relative to platinum. Unlike platinum, which has a higher ionisation potential and lower oxide stability, palladium exists mostly as an oxide with low specific activity for the oxidation of carbon monoxide and hydrocarbons. Furthermore, as discussed above, palladium is also known for its ability to react with sulphur dioxide, present in diesel exhaust gases, to form a stable sulphate which requires high temperatures to decompose.
Gold is a precious metal, which depending on the prevailing economic situation can be cheaper than platinum. Gold has been combined with palladium to provide an engine exhaust catalyst. WO2010/090841 A1 discloses a palladium gold catalyst which is supported on alumina.
EP 0602865 A1 discloses noble metal-metal oxide catalysts prepared by co-precipitation and their use to catalyse the oxidation of carbon monoxide and hydrocarbons in internal combustion engine exhaust gas. The metal oxide comprises one or more of ceria, zirconia, titania or stannic oxide, ceria being especially preferred. Noble metals disclosed include one or more of ruthenium, rhodium, palladium, osmium, iridium, platinum and gold.